Catalytic distillation is an emerging technology in which two distinct operations namely the catalytic reaction and the separation are carried out in one unit. Carrying out such a combination of steps in one unit has the potential to save energy, increase reaction conversion rate, improve product quality and reduce investment and operating costs.
The key part of developing the catalytic distillation technology is the design of internals for the catalytic distillation columns. The functions of column internals are to carry out both catalytic reaction and mass transfer simultaneously. In the past ten years, numerous patents have been awarded for the new design of the internals. They can be classified as follows:
1. Cloth Belt (U.S. Pat. No. 4,215,011). The catalyst is sealed in a cloth belt. The belt is then wrapped in open mesh knitted stainless steel wire. Liquid can penetrate into and flow out from the catalyst through the cloth. The wetted surface of the cloth belt provides the vapor-liquid interfacial area for mass transfer. The mass transfer efficiency for this kind of catalyst units is very low because of the low interfacial area. The effectiveness of the catalyst inside the belt may change from location to location because of the different liquid residence time. This reduces the overall system efficiency. PA0 2. Catalyst Container Held on a Tray (U.S. Pat. Nos. 4,536,373, 4,439,350, and 5,447,609). One type of this catalytic distillation unit consists of a normal distillation tray and a parallel array of rectangular tubes (troughs) filled with catalyst. The tubes (troughs) are constructed by metal screen and closed at both ends. The parallel tubes (troughs) may be supported above a distillation tray but should be located in froth zone when the column is in operation. The liquid flows across the tray in either parallel or right angle to the tubes (troughs). PA0 3. Packings. Random packings (U.S. Pat. Nos. 4,443,559, 5,057,468, 5,275,790, 5,262,012, and 5,189,001), e.g. Raschig rings, can be made of polymer catalyst such as ion exchange resins. The random packing in the column acts as both a catalyst and a mass transfer device. The random packing can also be made by rigid containers having a volume substantially smaller than the volume of reactive distillation column. The catalyst can be loaded into the container. Openings in the containers are provided to allow vapor and liquid passage into and out of the containers. The surfaces of the containers provide the necessary vapor-liquid contact for the distillation. The rigidity of the containers provides for the spacing of the structures and the necessary free space for the distillation. PA0 4. Tray Plus Fixed Bed of Catalyst (U.S. Pat. Nos. 5,130,102, 5,368,691, 5,013,407, and 5,026,459). In these catalytic distillation units, the reactive zone consists of alternate beds of catalyst and catalyst-free distillation zones. Passage ways may be provided for a vapor phase in the fixed bed. Distillation zones contain normal distillation trays and liquid distribution plates. PA0 5. Conventional Distillation Tray with Catalyst Placed in the Downcomer (U.S. Pat. Nos. 3,579,309, 5,277,847, 3,629,478, and 3,634,535). In these units, downcomers attached to the conventional trays are filled with catalyst which serve as a reaction zone and the trays act as a separation zone. PA0 6. Fixed Catalyst Bed with a Distribution Element (U.S. Pat. No. 5,523,062). In this arrangement, a corrugated plate having openings only at the peaks or valleys of the corrugation is used as a distribution element for the fixed catalyst bed. The plate and fixed bed are installed alternatively. The plate has little or no contribution to mass transfer. The liquid distribution to the beneath catalyst bed can be deteriorated if the plate is not properly installed. Radial mixing of liquid in the column is minimal. All these may cause the uncertainty in the operation of such a catalytic distillation unit. For the catalyst part, same disadvantages exist as packings stated above in (3).
Another type of this unit consists of a normal distillation tray and closed porous containers containing catalyst. The containers are installed above or on the distillation tray in the tower.
The structure of these units is usually quite complex. The catalyst quantity that can be loaded is limited. It is not suitable if the catalytic reaction step is slower because the reaction rate may not be matched with mass transfer rate.
Another type of this unit consists of a catalyst component and a resilient component intimately associated therewith. The resilient component has at least about 70% volume open space and being present with the catalyst component in an amount such that the catalyst distillation structure consists of at least 10% volume open space.
Structured Packings (U.S. Pat. Nos. 5,073,236, 5,235,102, and 5,348,710) are constructed with corrugated screen envelope with catalyst filled inside. Each envelope consists of two parallel layers of crimped screen, rough 6.25 centimeters (1 ft.) square, which are sealed at the edges. The envelopes are stacked and bound to make "bricks", which in turn are stacked to fill the column. The packing can also be in rigid, cellular monoliths or in a rigid, cellular monolith coated with a catalytically-active material.
Liquid holdup in the columns containing these internals is low. Therefore, it is not suitable to slow reaction. Like cloth belt, the effectiveness of the catalyst may change from surface to the inside because of the different liquid residence time. The cost of the internals could be very high.
The structure of these units are really complex. Liquid flow pattern may be far from plug flow. Therefore, the efficiency could be low.
In these units, the catalyst loading is limited by the downcomer size. It is not suitable for slow reaction processes. The downcomer may also cause a backup of liquid onto the tray.
It will be apparent that each or the different types of units have their advantages and disadvantageous. However, none the above described systems provide structures that may easily be adapted to couple a specific reaction with a desired separation i.e. for any given catalytic reaction there is a corresponding separation rate.